The present invention relates to fluid machinery for controlling a fluid pressure, and more particularly to a double screw rotor assembly for use in a vacuum pump, air compressor, water or oil pump, or any of a variety of fluid media to regulate the pressure of a fluid passing through.
FIG. 1 shows a double screw rotor assembly constructed according to U.S. Pat. No. 5,667,370 discloses another structure of double screw rotor assembly. According to this design, the meshed screw rotors 83 and 84 have same tooth height H", and the pitch is made gradually reduced in direction from the input side toward the output end 801 (P.sub.1 &gt;P.sub.2). Because of P.sub.1 &gt;P.sub.2, the volume of air chamber 830 or 840 is getting smaller during transmission, causing pressure to be relatively increased. Therefore, when compressed and transmitted to the output end 801, less pressure difference occurs on the output end, preventing a reverse flow of air and high noise. However, because of different pitches and pressure angles are defined at different elevations, the fabrication process of the screw rotors 83 and 84 is complicated, resulting in a high manufacturing cost.
FIG. 2 shows still another structure of double screw rotor assembly, which was filed by the present applicant under U.S. application Ser. No. 09/372,674. According to this design, two symmetric screw rotors 91 and 92 are meshed together and mounted in a compression chamber inside a casing, each comprising a spiral thread around the periphery. The thread has a tooth height H made gradually reduced from the input side 901 toward the output end 90. The threads of the screw rotors 91 and 92 define an uniform pitch P. The volumes of the air chambers 910 and 920 reduce gradually from the input side 901 toward the output end 90, so that pressure can be gradually increased during the transmission process, preventing a high consumption of power and high noise. Because an uniform pitch P is provided and the height H is made gradually reduced from the input side 901 toward the output side 90, the outer diameter D has the shape of an invertedly disposed cone, and the inner root diameter d has the shape of a regular cone. This design complicates the fabrication of the rotors 91 and 92.